My previous invention was limited to strength in tension or comprehension, achieving tension efficiencies compared with the tubular body itself of approximately 75%, being greater as the wall thickness of the tubular is increased. For example, a 75/8" outside diameter and 0.500 wall thickness liner connection of my previous invention, can only attain approximately 70% tension efficiency rating. As will be explained in this disclosure, my present invention obtains 90-95% tensile efficiency on the same tubular size and weight. The user of this invention saves money because a lighter grade tubular may be used to produce from a well and a longer tubular string may be used safely.
My previous connection, because of the particular design of the pressure seals, could not withstand a reverse pressure loading, that is, an internal burst pressure for that tubular size as rated by the American Petroleum Institute (API), followed by an external collapse pressure per API, and then finally subjected to another full internal burst pressure without failure of those seals. This invention is an improvement to my previous invention in that it will withstand API collapse pressure and show no leakage failure on subsequent internal pressure loading.
As a further improvement to my previous invention, this invention is much strengthened in resistance to torque. A source of damage of the previous connection is excessive torque in assembly. This connection features a torque stop located in the center of the joint which permits application of higher torque to the connection without damage. Locating the torque stop where the thickness of the relative members are nearly equal contributes to the higher torque capacity.
My previous connection offered protection of the primary seal surfaces but that protection is inadequate with moderately severe mishandling. My new invention provides maximum protection of the primary seal surfaces and the torque shoulder areas by locating said surfaces in the middle of the connection where the protection is greatest instead of on the ends as in my previous invention.
My previous connection offered thermoplastic seals with less tendencies for separating the male and female members upon assembly than other connections not using locking reverse load angle thread designs. Though the clamping action of the reverse angle thread profile in my previous design reduces the deflection of the members apart from the thermoplastic seal extrusion, some unwanted separation does occur. It occurs because the interferences on both ring and thread forms act concurrently, that is, in a dependent fashion. This separation reduces the effectiveness of the primary metal to metal seal in the immediate vicinity of said thermoplastic seals. There is also an increased amount of radial hoop stress generated by the radial separation of the male and female members. This invention remedies these faults simply by not requiring a thermoplastic ring. Such a ring may be used, however, but special techniques must be followed not covered by this invention to prevent compromising the primary seal.
My previous invention did not optimize the locking thread form that it introduced or specify preferred embodiments of the particular thread. My previous invention uses a variety of locking thread forms in which various design parameters such as radial and thread clearances, root and crest radii, and tolerance setting all may be varied, often to the detriment of the design. Consequently, modifications within the scope of this previous invention could compromise the invention's performance. This invention presents a thread element design that produces very low stress on engagement over the previous invention. Greater tolerance areas between mating threads are permitted without increasing the torque required to screw the parts together than in my previous invention. My new invention is more durable and reliable, being able to provide more repeated make-ups (assemblies) and break-outs (disassembles) without appreciable wear or change from originally manufactured tolerances than my prior invention.
Finally, my previous invention does not optimize low stress. The end of the female member (known as the face of the box) is relatively thin in cross section by design as is the end of the male member (otherwise known as the pin nose). Being thin, these ends easily deflect on being loaded and thus produce high values of radial and hoop stress. My new invention disclosed herein reduces such stress since it does not permit the end sections to be moderately stressed in the radial direction. The seal is situated in the middle of the connection where both inner and outer (male and female) sections are thickest and designed to be equally stiff with respect to one another. As a result the deflections are minimal and the stresses correspondingly low. Seal pressures are maximized. Torque shoulders are located in the same balanced vicinity so that they will contribute minimally to the stress in the connection.
There are a number of different sealing arrangements in the present art of these connections and are not known to be combined in the manner claimed herein. The NJO connection, licensed to Baker International and to others, features threaded sections with threads which do not decay on both the male and female connection members. Pressure seals are located at the end of the threaded sections. Because of using thread extremities for seals, it cannot achieve the same level of tension loading as my invention. Another connection manufactured by Hydril Company called the MAC is similar to the NJO in having two threaded sections, but it does not feature the threaded sections with decaying threads characteristic of my invention. To my knowledge none of these designs or others feature a cylindrical seal with decaying threads at both thread extremities.